This invention relates to a centrifugal separator of the type which can be used to extract heavy metal such as gold from a slurry containing the metals mixed with other materials of a lesser specific gravity.
Various designs of centrifugal separator have been proposed for this purpose including previous proposals by the present inventor set forth for example in U.S. Pat. No. 4,608,040. The device shown in the above patent has been very successful and operates in a very effective manner in various processing conditions. Two problems are encountered with this machine which limit its use in certain circumstances. In the first problem, the machine requires the introduction of additional water into the slurry as a backpressure through the holes in the wall of the bowl so as to improve the fluidization of the materials in the area between the rings or riffles on the bowl surface. In some cases this additional water is not available or provides additional processing problems. The second problem relates to the fact that the process is essentially a batch process and requires the machine to be shut down for a significant period of time for collection of the separated heavy materials.
Another proposal for a separator of this general type has been made more recently which provides a centrifuge member which defines a substantially cylindrical inner surface rotated at very high velocity. A dam member in the form of a ring having an edge of a radial extent slightly less than that of the cylindrical surface is mounted at one end of the cylindrical surface. The feed material in slurry form is supplied to the other end of the surface so that the material rotates with the centrifuge member and moves axially along the surface toward the discharge end of the surface. The dam at the discharge end causes a layer of the material to be formed on the inner surface of the centrifuge member of a thickness defined by the difference in radial extent between the dam and the surface. In practice this thickness is arranged to be of the order of one-eight to one-quarter inch. This layer of material acts as a separator so that the heavy materials are collected in the intersticies of the layer and are preferentially collected on the surface while the remaining material is discharged over the dam for collection.
This device has the advantage that it does not require any additional water added to the fluid. The separation technique is satisfactory and can provide a high concentration of the heavy materials or gold in many circumstances. It does however have a number of problems. Firstly the amount of material which can be collected on the surface before it is necessary to halt the process for discharge of the collected material is relatively small since the surface layer is only very thin. It is necessary therefore to halt the process at relatively high frequencies for collection of the separated material. Secondly the discharge of the material from the centrifuge is difficult to achieve even when the feed material is halted and the feed replaced by fresh water. The time period of the necessary shutdown is therefore relatively long.